Post-ganglionic sympathetic neurons originate in neural crest and migrate ventrally to establish the sympathetic chains. Having shown that differentiation of crest cells into sympathetic neuroblasts (sympathoblasts) requires influences from both somites and ventral half of spinal core, additional experimentation is planned to determine if each of these "inducing" tissues acts directly upon the crest cells or if they interact sequentially. Cephalic neural crest cells in situ, unlike trunk crest, fail to form sympathetic ganglia. Possible explanations for this, to be examined experimentally, are: (1) cephalic crest cells lack necessary environmental influences (e.g. somites); or (2) cephalic crest cells are developmentally different than trunk crest. To study regulation of neural crest cell migration and localization, attempts will be made to reveal the influences of extracellular materials and tissues located along the route of migration of the crest cells. Finally, the cytology of differentiating sympathoblasts will be described in greater detail. The techniques required in the proposed research include: (1) microsurgery for transplantation of crest cells and other tissues; (2) fluorescent microscopy for the localization of catecholamines in sympathetic neuroblasts; (3) heterospecific and radioactive labelling of cells, together with autoradiography and cytochemistry, in order to study migration and chemodifferentiation of the sympathoblasts; (4) histochemical and radioautographic procedures for the analysis of extracellular materials; (5) electron microscopy to follow cellular changes during sympathoblast differentiation.